I usually advocate buying in bulk and making big total profits over big profits per item. That's what usually works best for me. Consistent, steady, reliable profits over big, flashy numbers that don't usually add up to as much in total.
But sometimes you have to consider the low volume, high margin side of the equation too.
Take for example a mod or implant that's currently selling for 150 million.
Which is better?
1. Buying 10 at 140 million each and making roughly 8 million on each for 80 milliion total profit.
2. Buying just 1 for 75 million and making roughly 73 million on it.
If you still aren't sure consider this. While you do make 7 million more in total buying 10 you need 1.4 billion to buy them in the first place. All that ISK is tied up unavailable to use to purchase anything else until you both sell the items and post a new purchase order.
On the other hand buying just one cheaply only ties up 75 million, makes nearly as much in total AND leaves the other 1325 million available to purchase other things with.
The morale of the story is while buying and selling in bulk is what I normally advocate, there certainly are situations where you're better off working with low volume instead. Work with rules of thumb, not hard, inflexible rules and know when to change things up to take advantage of current conditions.
Of course the trick is managing to buy 1 for 75 million when everyone else is paying 140 million. That's mostly about location and transport and a little bit about product selection.